In a system in which a fluid is required to be supplied at a given controlled temperature, a three-way line valve may be used to divert the fluid to a heat exchanger or other device if its temperature differs from the required temperature. If the temperature is within the required supply range, the fluid is directed straight to the end use. For example, if the fluid is too hot, the valve will be used to divert it to a heat exchanger for cooling; similarly, if the fluid is too cold, the valve will be used to divert it to a heater.
Present systems of the kind referred to use electrically or pneumatically operated valves controlled by separate temperature sensors. Such valves are not self-contained but depend on external components and an external power supply for their control.